It is common practice to dry wood fibers, wood veneer, and plywood in ovens utilizing either heated air or steam flow as the drying means.
In typical steam dryer operations, the air flow can vary from about 5,000 to 10,000 SCFM. The drying causes large volumes of water vapor to be expelled from the wood and, together with the water, there are expelled organic materials such as rosin-type materials and terpenes. Such organic materials are not only vented to the atmosphere where they cause a substantial pollution problem but, in addition, the higher boiling fractions thereof, such as abietic acid, condense on the walls and exit of the smokestack or the dryer requiring extended maintenance because such buildup is a potential fire hazard.
Attempts to overcome this problem by the use of higher drying temperatures to degrade the organic materials or keep them from condensing have not been entirely successful due to the fact that such higher temperatures cause an undesirable darkening of the wood or wood fibers. In addition, apparatus and methods for handling the large amount of effluent gas from such dryers are not satisfactory in that they soon foul and themselves require extensive and repeated maintenance.